This invention relates to a magnetic clamp for a recording and reproducing disk, and particularly to a magnetic clamp operative suitably to clamp an optical recording disk medium on the spindle of the optical disk apparatus.
The majority of optical disk clamping devices for holding an optical disk on the spindle are based on the mechanical scheme as described, for example, in Japanese Patent Unexamined Publication No. 61-177671.
FIGS. 1 and 2 show a clamping magnet which is conceivably useful as a means of clamping an optical disk using the magnetic force. Provided on a base 26 are a motor 22, a spindle 28 which is the rotary shaft of the motor 22, and a disk table 29 formed on the top of the spindle 28. The optical recording disk apparatus comprises a spindle assembly 22, 28 and 29, an optical head 24 for recording or reproducing signals on the disk 23, and a rail 27 for coarsely moving the optical head 24 in the disk radial direction. The spindle 28 and disk table 29 are made of non-magnetic material such as stainless steel. Incorporated in the disk table 29 is a clamping permanent magnet 21, which holds the optical disk 23 by magnetically drawing a magnetic member 23A provided on the disk. Generally, optical disks such as a 5-inch disk have in their central section a formation of a magnetic (iron) disc with a diameter of about 15-20 mm and a thickness of about 1 mm, and the disks have a total thickness of about 5 mm in the central section. The disk drawing force is created by the magnetic flux which passes through the magnetic disc.
In order for the clamping magnet 21 to draw the disk 23 with a sufficient force, it should be a strong magnet and located closely to the disk 23 for the efficient exertion. The optical head 24 includes a fine actuator 25 which functions to move the lens up and down so that the recording/reproducing laser beam is always focused on the disk recording surface (i.e., auto-focus function) or to cause the focused laser beam to follow a track on the disk 23 (i.e., tracking servo function), or has both of these functions. The fine actuator 25, in most cases, is driven by a voice coil motor which uses a magnet and a coil. In order for the fine actuator 25 to focus the laser beam to form a small spot, it must be located sufficiently close to the disk 23. Accordingly, when the optical head 24 is positioned inwardly, i.e., approach to the center of the disk 23, the clamping magnet 21 and fine actuator 25 are in close positions, and unfavorably the optical head 24 is affected by the magnetic flux produced by the clamping magnet 21. The permanent magnet 21 is polarized alternately at a certain interval along the circumference of the ring-shaped magnetic plate, and its magnetic flux (magnetic field) 2lF draws the magnetic disc 23A which is secured at the center of the optical disk so that it is fixed on the spindle 28. When the clamping magnet 21 is polarized as shown in FIG. 2, for example, it produces a revolving magnetic field as the disk 23 rotates, which possibly causes the optical head 24 to vibrate in the disk radial direction.